1. Field of the Invention
The present invention deals with a lighting unit designed for functional similarity to an incandescent light source in which the principal source of light is an arc lamp supplemented by a standby filamentary light source, and which includes a compact "high frequency" power supply unit operating from a conventional 110 volt 60 hertz source.
More particularly, the present invention deals with measures designed to increase the level of standby illumination and to reduce the amount of electromagnetic interference during attempts to ignite the arc lamp.
2. Description of the Prior Art
The present invention is a product of efforts to produce an energy efficient and comparatively low cost replacement unit for the incandescent lamp. The incandescent lamp converts most of the electrical energy supplied into heat with only a small percentage, usually less than 10% being converted into visible light. With the cost of energy rising, a need has arisen for a lighting unit which converts electrical energy into light with greater efficiency. Recently, as disclosed in U.S. Pat. No. 4,161,672, smaller, low wattage, metal halide lamps having high efficiencies and light outputs comparable to home incandescent lamps have been invented. Such lamps are potential energy efficiency replacements for the home sized incandescent lamp provided that convenient low cost provisions can be made for standby illumination and for supplying the diverse electrical requirements for the two light sources.
The power supply of the present lighting unit represents an outgrowth of earlier high frequency power supplies in which a ferrite transformer, normally controlled for non-saturated operation and a transistor switch are significant elements. Such a power supply is disclosed in the application Ser. No. 156,229.
The foregoing power supply produces sustained high frequency energy starting with the high voltage ignition potentials for initiating the arc, continuing through the glow to arc transition when significant power at relatively low voltages is required by metal vapor lamps and terminating when the arc has stabilized at a low voltage signalling the initiation of "warm-up" and the onset of dc operation. Prior to warm-up, the high frequency energy is used both for the arc lamp and for energizing the filament providing standby illumination. In the normal start, the time that high frequency energy is requred for the arc lamp can be as short as one-tenth of a second and rarely longer than several seconds. At the same time, the circuit provides approximately 800 lumens of standby illumination. In the event that the arc lamp loses ignition in a hot condition requiring a "hot restart", then sustained high frequency application may continue for several minutes. However, should the arc lamp fail and refuse to ignite, the circuit would remain in the "pre-ignition" condition until a fuse or some other means terminates high frequency operation. Ideally, in the interests of minimal electromagnetic interference, the duration of the application of high frequency energy should be minimal under both circumstances. Standby illumination during the starting procedure should preferably be more nearly comparable to eventual illumination levels of the arc lamp, but must generally be compromised with the heat dissipation limits of the unit.